Recently, in the ink-jet printing industry, which has been significantly growing, the realization of high-performance ink-jet printers, the improvement of inks, and the like have markedly progressed, and it has become possible to easily obtain very fine images with high gloss which are substantially equivalent to film photos even in ordinary households.
In particular, the improvement of inks, such as a shift from known dye inks to pigment inks or a shift from solvent-based inks to aqueous inks, has been rapidly advanced in order to increase the image quality and reduce the load on the environment. At present, inks based on aqueous pigment inks are being actively developed.
Such inks have been required to have a higher level of performance year by year with the realization of high-performance ink-jet printers and the like. For example, scratch resistance at such a level that discoloration, degradation, and the like of printed images due to detachment of pigments caused by friction or the like when external forces are exerted to the surface of the printed images can be prevented and alcohol resistance at such a level that bleeding and discoloration of printed images are not caused when an alcohol contained in a detergent and the like is attached to the surface of the printed images have been strongly demanded in recent years without impairing good discharge stability and good storage stability of ink.
For example, the following ink-jet recording ink containing a pigment, an aqueous resin, and an aqueous medium is known as the above ink having excellent scratch resistance. In the ink-jet recording ink, the aqueous resin is a polyurethane resin obtained by a reaction of an organic diisocyanate with a diol having a polyoxyethylene structure, and the polyurethane resin has a carboxyl group and also has a particular acid value, a particular number-average molecular weight, and a particular amount of the polyoxyethylene structure (e.g., refer to PTL 1).
Images printed with the ink-jet recording ink have scratch resistance to a certain degree that, for example, the detachment of pigments due to rubbing between paper sheets can be prevented.
However, a higher level of scratch resistance is required with the expansion of the fields to which ink-jet printed matter is applied. In such circumstances, printed images formed using the ink-jet recording ink are sometimes still discolored, degraded, or damaged because of the detachment or the like of pigments, for example, when strong external forces are locally exerted. There is also a case where, when an alcohol adheres to the surface of the printed images formed using the ink-jet recording ink, floating, bleeding, and the like are caused on the printed surface.
As described above, an ink-jet printing ink that can form printed images having excellent scratch resistance and good alcohol resistance without impairing good discharge stability and good storage stability of ink has been demanded from the industrial world. However, such an ink-jet printing ink and a binder for ink-jet printing ink that can be used for the production of the ink-jet printing ink have not been found yet.